Inorganic photovoltaic devices tend to be expensive to manufacture and thus are generally not sufficiently cost-effective for widespread deployment as an alternate electricity producing method to conventional electricity production systems. In addition to the high cost, inorganic photovoltaic devices are typically heavy and brittle and require significant infrastructure for installation. As a result, purchasing inorganic photovoltaic devices requires a significant amount of time before the investment produces a return. Organic photovoltaic devices (OPVs) hold the potential to be much less expensive and include functional advantages such as colour tunability and the potential for flexible products. Despite the significant advantages of OPV technology, OPVs currently lack sufficient performance and stability for successful commercialization. As a result, there is a need for more efficient and lower cost organic photovoltaic systems.
OPVs typically operate by absorbing sunlight and creating energetic particles known as excitons. These excitons consist of strongly interacting pairs of electrons and holes, which are treated as single bound particles. In order to extract electrical energy from excitons, the excitons must first migrate to an interface that is capable of separating their component charges. Once separated, the electrons and holes are transported to electrodes where they are extracted from the photovoltaic device to produce an electrical current.
Polymer OPVs typically employ a two-component photoactive layer (PAL) consisting of an electron-donating conjugated polymer and an electron-accepting fullerene, structured in a disordered bicontinuous interpenetrating network known as the bulk heterojunction (BHJ). A widely researched polymer-based OPV device employs a photoactive BHJ of regioregular 2,5-diyl-poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In this excitonic polymer OPV configuration, the absorption of light generates bound electron-hole pairs (i.e., excitons) that ideally dissociate into free charge carriers at the P3HT-PCBM (donor-acceptor) interface. Once the charge carriers are dissociated, they must migrate to the electrodes where they are collected and delivered as useful current.
In order to increase cell efficiency, interfacial modifiers (IMs) are applied at each of the electrodes, and these IMs reduce the energy required to extract charge carriers from the device. IMs that accomplish this task more effectively are expected to improve electrical current output, increase open-circuit voltage and render the devices more efficient. The use of IMs are therefore important to the fabrication of efficient organic electronic devices.
The most common IM used in organic electronics is sodium poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) [(PEDOT:PSS)−Na+]. (PEDOT:PSS)−Na+ is an organic polymer blend of cationic, conducting PEDOT that is charge over-compensated by anionic, insulating PSSNa. This polymer blend is usually applied in organic electronics as a hole collecting interfacial modification layer on indium tin oxide (ITO) due to its stable and high work function. A (PEDOT:PSS)−Na+ interfacial layer also leads to a smoother electrode with improved ohmic contact with the active layer, enhanced hole collection, increased open-circuit voltage (Voc), as well as improved areal electrical uniformity in completed OPV devices. In fact, anionic (PEDOT:PSS)− has been deposited with cationic poly(p-xylene-α-tetrahydrothiophenium) using electrostatic layer-by-layer (LbL) assembly to modify ITO to control electron leakage in polymer light-emitting diodes.
Despite its ubiquity, PEDOT:PSS offers little in the way of control and/or tunability of the work function. In addition, it is known that PEDOT:PSS can contribute to the degradation of OLEDs and OPVs as a result of its high acidity which can promote ITO etching and mobile byproduct indium ions and an excess of PSSNa which can migrate throughout the BHJ and react with components of the photoactive layer. Finally, PEDOT:PSS is not applicable for modifying ITO for inverted solar cells. Operating a polymer BHJ OPV device in an “inverted mode” where electrons are extracted from the transparent electrode and holes from the reflective electrode, generally requires significant tailoring of the electrode work functions using interfacial modifiers, but is often advantageous with respect to performance stability, design flexibility, and compatibility with stacked and/or tandem architectures.
The work function value of electrodes in organic devices such as OPVs, organic light emitting diodes (OLEDs), capacitors, and organic thin film transistors (OTFTs) is important for the efficient transport and extraction of generated charges. If there is poor energy alignment between the work function of the electrode and the energy levels in the PAL, electrons and/or holes can be blocked limiting the amount of available current from the device. As such, the ability to tailor the work function of the electrodes is important in optimizing device fabrication. Although there are a number of IMs presently known that are capable of modifying the native work function of an electrode, they require various heating and annealing steps and fail to offer much control over the resulting work function. Typically, a large number of materials are synthesized and subsequently probed in order to determine the appropriate work function. This approach is timely and expensive with little guarantee of success.
The lifetime stability of OPVs is often limited and rigorous studies have identified several key degradation mechanisms such as photooxidation and morphology evolution in the structure of the multilayer device (1-4). Conventional donor polymers such as 2,5-diyl-poly[3-hexylthiophene)] (P3HT) are often prone to oxidation in air when illuminated, (5, 6) while a polymer with a deeper set highest occupied molecular orbital (HOMO) energy level is less prone to oxidation, and in turn contributes to an increased stability in the OPV (7, 8, 9-11). Additionally, interfacial buffer layers are known to affect the lifetime of OPVs (12). Specifically, the common hole transport interfacial modifier, PEDOT:PSS, can contribute to the degradation of organic light emitting diodes and OPVs as a result of: i) its high acidity which can promote ITO etching and mobile byproduct indium ions (13) and ii) an excess of poly(sodium 4-styrenesulfonate) (NaPSS) which can migrate throughout the bulk heterojunction (BHJ) and react with components of the photoactive layer (12).
Effective modulation of the electrode work function requires precise tuning of the interface between the electrode and the photoactive layer in OPVs. As such, there is a need for methods whereby the work function of the electrodes can be precisely tuned to match their energy levels with those in the PAL. In addition, there is a need to limit the amount of material required to reach a desired work function value.
To date, surface modification has been successfully demonstrated using only inorganic or surface-functionalized inorganic materials. The tailoring of the work function of ITO with purely organic polymer/small molecule coatings for organic electronic devices is largely unexplored and is attractive from several standpoints. For example, many organic polymers can be tailored to match the electronic, morphological and physical requirements for improved device performance. In addition, solution-processable polymers are desirable from an industrial perspective as costly vacuum deposition equipment is avoided, and in a further refinement, water-soluble polymers are particularly advantageous as organic solvents are relatively expensive and environmentally harmful. The quality and uniformity of polymer coatings is often higher than those of inorganic counterparts and the use of these coatings leads to improved mechanical flexibility and robustness of the overall device compared to devices made from inorganic materials. In addition, polymers are amenable to electrostatic multilayer approaches whereby nanoscale material can be deposited one layer-at-a-time in order to fabricate multilayer films of precise thickness. The electrostatic multilayer assembly of water-soluble interfacial modification polymers on electrode surfaces is therefore an attractive complement.
A variation in the structure of the same material, as opposed to testing a large number of potential compounds, would offer a powerful and efficient compliment to device optimization for organic electronics. A simple approach to electrode work function modulation, using only a few available materials, would allow a large number of PALs to be matched with the electrode.
A review of the prior art reveals various known polymers and multilayer fabrication methods used to affect the electronic properties of substrates. Buriak et al. (Adv. Funct. Mater. 2010, 20, 2404-2415) discloses the synthesis of a cationic and water-soluble polythiophene ([poly[3-(6-pyridiniumylhexyl)thiophene bromide]) and its use in hybrid coatings on indium tin oxide (ITO). Rubner et al. (Macromolecules 1997, 30, 2712-2716; J. Appl. Phys. 79 (10) 15 May 1996) discloses the fabrication of light-emitting diodes based on self-assembled multilayers of poly(phenylene vinylene) as well as multilayer fabrication using polyaniline. Friend et al. (Nature vol 404 Mar. 30, 2000, 481-494) discloses molecular-scale interface engineering for polymer light-emitting diodes using poly (p-phenylene vinylene) (PPV) and poly (p-xylylene-a-tetrahydrothiophenium) (PXT) along with PEDOT:PSS. Fermin et al. (Chem Phys Chem vol 5 Dec. 19, 2003, 571-575) discloses the change in work function that results from electrostatic layer-by-layer assembly of nanoparticles and polyelectrolytes. Hosono et al. (Physical Review B vol 83 2011 115435) teaches the change in work function of Nb doped SrTiO3 substrates covered with MgO. Li et al. (J. Phys. Chem. B. vol 105 2001 10022-10028; J. Phys. Chem. B. vol 104 2000 11195-11201) teaches the change in surface electronic properties of self-assembled multilayers on conductive oxides as well as the change in electronic potentials of self-assembled monolayers utilizing metal phthalocyanine and oligomeric viologen on conductive substrates. Salaneck et al. (J. Phys. Chem. C, 2007, 111 (6), 2724-2729) discloses layer-by-layer deposition of copper phthalocyanine from aqueous solution and the resulting electronic structure. Electrostatic layer-by-layer (eLbL) self assembly of the small molecule PTCDI+ with anionic surfactants or polyelectrolytes has been demonstrated in several papers (Langmuir vol 25 2009 1188-1195; J. Mater. Chem., vol 19 2009 2356-2362; Langmuir vol 24 2008 43-48; Adv. Funct. Mater. vol 18 2008 1890-1897.